1. Field of the Invention
The present invention relates to a duty ratio control circuit apparatus for controlling the duty ratio of an output signal to be 50% or an arbitrary value.
2. Description of the Related Art
Conventionally, when a timing signal for controlling a system is formed by inputting an output from a quartz oscillator to a timing circuit, a fundamental clock for driving the timing circuit is required to have a more accurate duty ratio as the operation speed of a system is increased. In a normal system, a fundamental clock having a duty ratio of 50% is required. For this reason, in a conventional system, a quartz oscillator is operated in advance at a frequency of an integer multiple of a desired frequency f.sub.0, generally a frequency 2f.sub.0, twice the frequency f.sub.0, and the output from the quartz oscillator is frequency-divided by a 1/2 frequency divider to have a duty ratio of 50%. The resultant output is then input, as a fundamental clock, to a timing circuit. In this manner, a signal having a high frequency is generated by the quartz oscilator and is frequency-divided to form a fundamental clock having a desired frequency for the following reason. Although the frequency of a signal obtained by the quartz oscillator is generally controlled with high precision, its duty ratio or waveform considerably deviates from an ideal state. Hence, such a signal cannot be directly used as a fundamental clock to be supplied to the timing circuit. In addition, a quartz oscillator of several 10 MHz or more is generally expensive and is susceptible to a mechanical shock. Besides, such an oscillator is difficult to obtain.
A circuit disclosed in RCA Review Vol. 41 March 1980, "Design and Performance of a CCD Comb Filter IC" PP. 36-PP. 40 is known as a circuit for realizing a 50% duty ratio, which is incorporated in an IC device. This circuit requires an external capacitor as a filter and can receive only signals having specific waveforms such as triangular waves and sine waves, i.e., cannot be applied to rectangular waves and the like.
In the conventional method, a circuit using a PLL circuit including a VCO (voltage-controlled oscillator) may be employed. In this case, however, the circuit arrangement becomes very complicated, and a problem is posed in terms of stability of circuit operation. Therefore, such a circuit is not preferable.
In the conventional method in which a quartz oscillator is oscillated at a frequency of an integer multiple of a desired frequency, since the frequency of the quartz oscillator or the like is high, the current consumption becomes excessively large. In addition, a frequency dividing circuit for receiving an output from the oscillator must be designed to cope with variation in duty and amplitude of an input signal, which is caused by the manufacturing process, and changes in duty and amplitude due to environmental conditions. Therefore, such a conventional method tends to greatly decrease the margin.
In addition, the circuit disclosed in the above-mentioned RCA Review requires capacitors having large capacitances as a capacitor for a low-pass filter, an input coupling capacitor, and the like, and these capacitors must be arranged outside the IC. Besides, this circuit can receive only an input signal which can be converted into a signal having a duty ratio of 50% by adjusting its DC level, such as a sine wave or a triangular wave, but cannot receive a rectangular wave. Furthermore, a circuit using a PLL circuit requires a large number of external components and incorporates potentially floating nodes, and hence is susceptible to noise. Besides, it takes a very long period of time to stabilize the circuit. Therefore, such a circuit is not preferable. Moreover, since the oscillation frequency of the VCO is dependent on power source voltage, temperature, manufacturing variation, and the like, it is very difficult to obtain stable oscillation at several 10 MHz or more. Therefore, such a circuit is very difficult to realize.
Under the circumstances, a strong demand has arisen for a duty ratio control circuit apparatus which can operate an oscillator at a frequency equal to that of a signal input to a timing circuit, can satisfactorily convert a rectangular wave as well as sine and triangular waves into a signal having a duty ratio of 50%, and can be realized in an IC without requiring any external elements.